Generally, in High Voltage Direct Current (HVDC) systems, Alternating Current (AC) power produced in a power plant is converted into DC power and the DC power is transmitted, and a power receiving stage re-converts the DC power into AC power and supplies the AC power to a load. Such an HVDC system is advantageous in that power may be efficiently and economically transmitted via voltage boosting, and in that connections between heterogeneous systems and long-distance high-efficiency power transmission are possible. Further, a Static Synchronous Compensator (STATCOM) is a kind of Flexible AC Transmission System (FACTS) device, and denotes an electric power electronics-based compensator, which is connected to a power system and is used to increase power transmission capacity and to maximize the usage of existing facilities. Such a STATCOM system is advantageous in that power systems are compensated in parallel using a voltage-type power semiconductor device, thus stabilizing the systems by maintaining voltage at a constant value.
An MMC may be connected to an HVDC system or a STATCOM. In such an MMC, multiple sub-modules are connected in series. In the MMC, a sub-module is one of the most important components. Therefore, in order for sub-modules to operate normally in various environments, there is a need to stably supply power to the sub-modules. Further, in the MMC, sub-modules become a current path through which voltage is converted and power is transmitted. Since loss occurring during the operation of sub-modules negatively influences the efficient operation of the sub-modules, efforts to minimize such loss have been continuously made.
FIG. 1 is an equivalent circuit diagram of an MMC, and FIG. 2 is a circuit diagram of a conventional power control apparatus for MMC sub-modules. As is well-known to those skilled in the art, an MMC is composed of one or more phase modules 1, and individual sub-modules 10 are connected in series in each phase module 1. Further, DC voltage terminals of each of the phase modules 1 are connected to positive (+) and negative (−) DC voltage buses P and N, respectively. A high DC voltage is present between the DC voltage P and N buses. In each sub-module 10, two connection terminals X1 and X2 are formed.
A conventional power control apparatus 20 for MMC sub-modules converts a high voltage (about 2 to 3 kV) on the P and N buses into a low voltage (about 300 to 400 V) so as to supply the power required to operate the sub-modules. Here, in order to maintain high reliability depending on the characteristics of the HVDC system, the coupling of resistors R and a Zener diode Z is used. For example, current is limited using specific resistors R1 and R2, among multiple resistors R1 to R3 connected in series between the P and N buses, and the high voltage is converted into the low voltage using the Zener diode Z.
However, the conventional power control apparatus 20 is problematic in that loss occurs due to heat generated in the resistors R1 and R2 for current limiting, and such heat generation may negatively influence the overall operation of the power control apparatus because it is closely related to the reliability of elements. Thus, there is inconvenience in that a heat dissipation plate for preventing heat generation must be separately attached.
Further, the sub-modules of the MMC connected to the HVDC system accommodate voltages falling within a very wide range (0 to 3 kV), and must drive the MMC by combining the voltages, and thus the power of the sub-modules must be normally supplied at a voltage of 800 V or less. For this reason, if current-limiting resistors R1 and R2 are selected so that the control power is normally output in a range of 800 V, and the input voltage is increased up to 3 kV, high current flows through the resistors R1 and R2, thus increasing loss and resulting in heat generation.
At the same time, most of the increased current flows into the Zener diode Z, so that high heat is generated in the Zener diode Z, thus greatly deteriorating the reliability of the apparatus. The reason for this is that the resistance of the Zener diode Z is lower than the line resistance of the resistor R3.
In the case of the current-limiting resistors R1 and R2, heat dissipation may be smoothly performed using a heat dissipation plate, but, in the case of the Zener diode Z, a problem arises in that it is difficult to attach a heat dissipation plate or the like, for the reason of increased volume or the like.
Therefore, in the field of this art, there is required the development of technology for a power control apparatus, which enables power control to be stably performed while minimizing the loss of current-limiting resistors without requiring the installation of additional elements in the sub-modules of an MMC connected to an HVDC system.